1. Field of the Invention
This invention relates generally to data timing sharing and recovery in a communication system, and even more particularly to derivation of precise TDMA uplink timing across multiple satellite asynchronous Digital Video Broadcast (DVB) transport streams.
2. Description of the Related Art
Using satellites for Internet and Intranet traffic, in particular multicasting of digital video through use of DVB and two-way broadband communication has recently received a great deal of attention. There are a number of applications using satellites in one or two-way data communications, and each presents unique timing and transmission problems which must be considered. Satellites can help relieve Internet congestion and bring the Internet and interactive applications to countries that do not have an existing network structure, as well as provide broadband interactive application support.
In a typical broadcast mode, geosynchronous satellites relay a signal from a single uplink station to a number of receivers within the “footprint” of the satellite. The satellite system covers a footprint, which could, for example, represent all or a portion of the continental U.S. When the signal carries packetized digital data, a geosynchronous satellite is an excellent mechanism for carrying multicast data, as a multicast packet need only be transmitted or “broadcast” once to be received by any number of remote receivers. Such a signal, by carrying both unicast and multicast packets, can support both normal point-to-point and multicast applications.
As one means of using satellite technology in this growing field, very small aperture terminals (VSATs) provide rapid and reliable satellite-based telecommunications between an essentially unlimited number of geographically dispersed sites. VSAT technology has established effective tools for LAN internetworking, multimedia image transfer, batch and interactive data transmission, interactive voice, broadcast data, multicast data, and video communications. The emergence of VSAT technology has provided a practical solution for broadband delivery. Using a system of deployed satellites in conjunction with the necessary ground-based infrastructure and VSAT terminals, users can potentially transmit and receive video, audio, multimedia, and other digital data hundreds of times faster than over conventional phone or terrestrial data lines.
The Internet Protocol (IP) is the most commonly used mechanism for carrying multicast data. Examples of satellite networks capable of carrying IP Multicast data include Hughes Network System's Personal Earth Station (PES) VSAT system and Hughes Network System's DirecPC® system. Combining VSAT delivery with standards-based IP multicast ensures users a less expensive and more flexible approach to achieving high-quality, real-time broadcasting.
As for digital TV transmission, MPEG-2 emerged as the digital entertainment TV compression standard (ISO 13818) for transmission media such as satellite, cassette tape, over-the-air, CATV, and new broadband multimedia data and interactive services wherein MPEG-2 packets are used as “data containers”. The MPEG-2 system standard simply defines a packet structure for multiplexing coded audio and video data into one stream and keeping it synchronized. Although the MPEG-2 standard does not prescribe which encoding methods to use, the encoding process, or encoder details, the standard does specify a format for representing data input to the decoder, and a set of rules for interpreting these data. Video can thus be encoded using inexpensive MPEG standards-based encoders that encapsulate the MPEG packets in IP multicast frames.
MPEG-2 defines two types of streams—the Program Stream which includes the packet structure above, and the Transport Stream, which offers robustness necessary for noisy channels, as well as the ability to include multiple, asynchronously multiplexed programs with independent time bases in a single stream. The Transport Stream is well-suited for delivering compressed video and audio over error-prone channels such as a satellite transponder. However, the MPEG-2 specification does not provide all the information necessary to ensure interoperability, data broadcasting, and delivery scheduling in a TV system.
In response to this need, DVB standards have been developed and published by the European Telecommunications Standards (ETSI), and have been globally adopted. DVB is fundamentally an MPEG-2 based system, which provides the basis of DVB video, audio, and transport across a variety of media such as satellite, cable TV, broadcast, etc. For this reason, DVB has defined a set of implementation guidelines for MPEG-2 in DVB which cover the minimum requirements for interoperability for baseline standard definition television (SDTV), high definition television (HDTV), and DVB Integrated Receiver Decoders (IRD). Data broadcasting is a key application of digital TV, and DVB has taken elements of MPEG-2 Digital Storage Media—Command and Control (DSM-CC) and produced specifications and guidelines which now provide the basis for most data broadcast applications around the world.
MPEG-2 was chosen as the basis for DVB source coding of audio and video, and for the creation of Program Elementary Streams and Transport Streams at the systems level. However, MPEG-2 standards are generic and are considered by the industry to be too wide in scope to be directly applied to DVB. Accordingly, industry guidelines have been established to restrict MPEG-2 syntax and parameter values, as well as suggesting preferred values for use in DVB applications to ensure interoperability across different media, a requirement which is frequently needed in the complex signal distribution environment. The core of DVB is its series of transmission specifications, including the DVB-S satellite transmission standard, based on QPSK or Offset QPSK (OQPSK), which is now the defacto world satellite transmission standard for digital TV applications.
Satellite DVB technology and the Internet Protocol (IP) have thus necessarily converged (“IP/DVB”) to allow users transparent access to a variety of broadband content, including live video, large software applications, and media-rich web sites. The borders between digital video broadcasting and computers have necessarily blurred —TV broadcasters transmit data, businesses broadcast multimedia applications, and even the most remote user can use interactive communications. From the outset of DVB, interactive applications have been perceived as being the cornerstones of the new generation of digital television. One of the strengths of DVB technology lies in the fact that it enables the point-to-multipoint transmission of very large amounts of data at high data rates while securely protecting against transmission errors. Such data may include audio and video but, in many applications, the data will be large files or other forms of generic information.
In support of these developments, VSAT systems, such as the Personal Earth Station mentioned above, allow commercial users to access one of a generally limited number of satellite return channels to support two-way communication. The choice of return or inbound channel is usually restricted to only a few of the possible channels preconfigured by a combination of hardware and/or software limitations. Other consumer-oriented hybrid systems, such as DirecPC® Turbo Internet, may use a dialup modem or other terrestrial link (as well as other non-satellite media) to send HTTP requests to the Internet, and may receive responses either via the outbound satellite channel, or a dialup modem connection. Some commercial systems may use a VSAT system terminal for Internet access to receive HTTP responses via the outbound satellite broadcast channel, and to send HTTP requests to the Internet through a VSAT inbound channel. Unfortunately, as these systems are mass-marketed to consumers and the number of users increases, the generally limited number of inbound channels can experience congestion and reduced user throughput as a result of an increasing number of users competing for a finite number of inbound satellite channels. The potential benefits that VSAT technology bring to consumers in the area of broadband delivery are necessarily diminished.
FIG. 1 partially depicts one-way satellite broadcast system 100 wherein One-Way NOC 110 transmits DVB transport stream 120 to through satellite 130 to multiple remote users 150 (1 to n). Each remote user 150 has a receiver (RCVR) 140 which receives and demodulates the data contained in DVB transport stream 120. One-Way NOC 110 may also provide and receive information to/from the internet or an intranet through gateway 160. The return link from remote users 150 to One-Way NOC 110, e.g. a terrestrial line, is not shown.
As the use of two-way satellite networks has expanded into the consumer market, industry has further pursued internetworking of multiple satellite-broadcast networks and their associated independent inroute (“inbound”) or uplink channels. As the market expands, the number of possible uplink users further increases, and the previous approaches to allocation of return channels to users in fixed, predetermined groups necessarily requires additional hardware and system complexity in order to accommodate the increased uplink demand. Further, this approach becomes increasingly inefficient both in terms of hardware allocation, cost, and uplink channel utilization, since many of the available groups of uplink channels may be either heavily or lightly loaded or subject to load imbalance relative to other inroute groups because of each user being hard-configured for access to a specific inroute channel, or to only a limited number of channels.
Slotted-time uplink channels are commonly used and may be based on a Time-Division Multiple Access (TDMA) approach, wherein precise system timing is necessary to allow multiple users access to the necessary bandwidth and ability to transmit information in a multiplexed fashion on the return channel. TDMA allows a number of users to access a single radio frequency (RF) channel without interference by allocating unique time slots to each user within each channel. In TDMA, access is controlled using a frame-based approach. Transmissions are grouped into frames, with a frame synchronization (“sync”) signal usually being provided at the beginning of each frame. Following the frame sync, there are a number of time “slices” within the frame used for a burst transmission. In the simplest case, one time slice is allocated to each of the users having the need to transmit information. In more complicated systems, multiple time slices are made available to users based on transmission need or a prioritization scheme. After all time slices have elapsed, another frame synchronization signal is transmitted to restart the cycle. Thus, the frame sync serves as a system time reference that provides a common transmit timing source to each uplink user who transmits in a burst during a pre-assigned time slot.
TDMA requires a method for precise timing of the epochs of burst transmission to reduce burst overlap and consequent “collisions” of different users' transmissions. Providing a common time reference for a limited number of remote network receivers receiving a single downlink or broadcast beam and sharing a limited number of uplink channels is relatively easy to accomplish, particularly when transmission and reception delays between the network control and the various users are well-characterized. For example, if synchronous operation is used, i.e., where the symbol rate of the digital transmission signal is precisely a multiple of the TDMA frame frequency, the TDMA frame rate can be locked to the system symbol clock at the network hub or earth station, and remote users can derive the frame rate from the recovered symbol timing.
However, frame timing sharing is more difficult with the evolution of multiple-beam satellites, and when sharing a larger number of different inroute or uplink channels among a large number of users. These users may be receiving different asynchronous broadcasts transmitted either through the same or different transponders on the same satellite or even on different satellites. Asynchronous digital transmissions have a symbol rate which is not a multiple of the TDMA frame rate. Establishing a common uplink transmission time reference for each of the users is more difficult due to the variety of delays and transmission paths in use, as well as the asynchronous nature of the broadcasts.
Several potential solutions for symbol timing recovery are available when asynchronous broadcast transmissions are used. For example, Global Positioning System (GPS) based timing, packetized elementary stream timing for Program Streams, or MPEG-2 Program Clock Reference (PCR) timing for Transport Streams may be used to synchronize a system. However, each of these solutions has relatively high-cost because of the additional processing and hardware requirements, including additional equipment at each of what could be a large number of remote user sites.
Currently, single, low-cost timing sources for sharing both frame and symbol timing throughout a communication system, particularly across multiple asynchronous transport streams is not available.
What is needed, therefore, is a relatively low-cost, accurate, and reliable system and method for sharing synchronized uplink data frame and symbol timing across a large network of geographically dispersed users receiving information across multiple transport streams, carriers, or satellites, without the necessity of involving multiplexing and modulation equipment. What is further needed is a system and method which solves the timing and uplink access problems associated with an increase in the number of users in the system, and which eliminates the need for major modifications or additions to network components required to transmit and receive the data.